Insights Into the Early Volatile History of the Earth From the Geochemistry of Iodine and Xenon

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1009 Geochemical Modeling (3610, 8410), 1025 Composition Of The Mantle, 1038 Mantle Processes (3621), 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)

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Evidence that liquid water was stable on the Earth before 4.4 by ago comes from studies of very old zircons. This evidence seems at odds with the giant impact model of the Moon's formation, which should have resulted in melting of a large portion of the Earth's interior and the formation of a deep magma ocean. The stabilization of a primitive crust, and the attainment of surface temperatures allowing liquid water stability would have to have occurred within about 100 my of the onset of solar nebula condensation. Xe129, produced by the decay of now extinct I129 (t1/2 = 17 my), provides a tracer for volatile processes in the earliest history of the Earth. Catastrophic outgassing of the mantle as proposed to explain the Xe129/Xe132 ratios of MORBs (1.014) and the atmosphere (0.985) could have happened in the context of a post-giant-impact magma ocean only if it occurred on a timescale of 100 my or less. To assess the effects of outgassing of an early magma ocean on the I/Xe systematics of the Earth's mantle, we consider a simple two-reservoir (MORB-source mantle and atmosphere) model with partial melting of the mantle occurring in equilibrium with residual minerals followed by eruption and degassing. Experimentally determined I and Xe mineral/melt partition coefficients and silicate-melt solubility values for I and Xe are employed in mass balance calculations based on this two-reservoir model. The amount of I/Xe enrichment in the MORB-source mantle and the extent of melting and outgassing needed depend on the timing of outgassing. If it was completed by 50 my following solar nebula condensation, then the MORB-source mantle needs to have been at least 50% molten and 90% outgassed. If it was completed by 100 my, then the mantle must have been at least 95% molten and 99% outgassed. So, the longer the timescale of magma ocean outgassing, the more extensively melted it has to be. This result accentuates the necessity for a quick cooling of the magma ocean and the early stability of liquid water and illustrates the difficulty of fractionating I from Xe by purely magmatic processes. However, water is extremely effective at fractionating I from Xe. Once liquid water became stable at the Earth's surface, outgassed I could be recycled hydrothermally into the oceanic crust and subducted back into the mantle, which was, by now, partially degassed of Xe. Subsequent decay of I129 would give a MORB-source mantle with an elevated Xe129/Xe132 ratio relative to the earlier outgassed atmosphere.

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